In addition to its other functions in vesicle trafficking, the trans-Golgi network is the parent organelle from which components of secretory granules in neuroendocrine cells and melanosomes in melanocytes originate. These two specialized storage organelles have remarkable similarities that heretofore have been under-appreciated. In particular, the organization of both compartments hinges on the assembly of a luminal protein matrix that forms within a specialized ionic milieu, and this is hypothesized to serve as a fundamental mechanism for protein retention in such compartments. This application proposes to explore relationships in the biogenesis of these two post-Golgi storage organelles. Thus, new content proteins of the melansomal matrix will be identified, with the intent to ultimately characterize the importance of protein/protein interactions to storage. The cDNAs encoding these proteins will also be expressed in neuroendocrine secretory cells that maintain in storage granules a specialized lumenal environment highly analogous to that of melanosomes but with a very different protein content. I propose to initiate the heterologous expression studies immediately with MMP115, the one melanosomal cDNA that has so far been cloned which does not encode a single-spanning membrane protein, and is believed to reside entirely within the melanosome lumen. Moreover, I plan to develop an assay in which the de novo budding of immature melanosomes from the TGN is reconstituted in vitro using a permeabilized melanocyte cell system. I believe that the studies proposed in this application will provide important insights into general mechanisms of protein sorting and storage in post-Golgi compartments.